To understand the Navy’s success — or lack of — in closing its at-sea gaps, you have to understand how the service measures manning.

Personnel officials do so using two terms: fit and fill.

Fit is the percentage of billets at sea filled with sailors of the right rating and the correct paygrade.

Fit is best, but the Navy is unlikely to ever hit 100 percent, so officials have built in some wiggle room. Fill is the percentage of at-sea billets filled with sailors of the right rating, but not necessarily the right paygrade. Leaders have the power to fill billets with sailors as much as two paygrades below what they need or one paygrade above.

The Navy’s goal is to reach a fill level of 95 percent and a fit level of 90 percent by Sept. 30. So how is it doing?

As of June 26, the Navy still had 9,511 empty billets, or a fill of about 93 percent. The fit percentage is at about 88 percent, with 16,100 billets that still need sailors of the correct paygrade and rating.

“We will always have shortfalls, but we can continue to pick away at them and I think we’ll be just about there in October,” said newly retired Vice Adm. Scott Van Buskirk, the former chief of naval personnel. “If it isn’t that exact number, we’ll be quite close — and also it depends on where you look, because we have different priorities for our people in different places.”

While overall, the Navy is nearing its goals, it can vary dramatically by ship. Fleet Forces Command calls for U.S.-based ships to have at least 90 percent in both fit and fill when they deploy, but as of March, even with last-minute manning actions, ships were deploying with 85.1 percent fit and 88.2 percent fill.

The submarine force was given the ability three years ago to send units out 95 percent and 85 percent fill and fit respectively. And at the beginning of this fiscal year, the surface force, after adding back billets in a fleetwide buyback program decided to temporarily allow its units to deploy with 85 percent fit and 87 percent fill.

As for forward-deployed ships, they’re supposed to be at 95 percent for both metrics, but currently they’re at 94 percent.

“Today our focus is on improving sea duty manning,” said the new CNP, Vice Adm. Bill Moran, who replaced Van Buskirk on Aug. 2, in a statement. “Continuing to bring greater stability and certainty to the sea duty detailing and manning processes improves fleet readiness, sailor quality of service and overall predictability. It’s about getting the right sailor with the right skill set and leadership experience to the right sea duty billet at the right time.”